Blender Nodes in Editor doesn't show preview Image
You need to render the scene before anything shows on the compositor. HitF12 or press Render.


To have the Viewer Node's image as background behind the node tree you need to enable the backdrop button:



If you press N on the Node Editor's window  you'll see Controls for size and placement of the backdrop:



Or you can zoom in and out by pressing V or ALTV

You can also choose to see the Composite Node or Viewer Node in a UV/Image Editor window.
Selecting Render Result will display the Composite Node